• Limitations of Reynolds Averaging in Forced Skewed Turbulent Mixing Layer

      Wygnanski, Israel J.; Yamauchi, Atsushi; Little, Jesse C.; Tumin, Anatoli (The University of Arizona., 2019)
      An experimental investigation aimed at studying the three-dimensional character of a turbulent mixing layer created downstream of a swept-back wing that served as a splitter plate and a screen that reduced the velocity in the test section on the lower side of the wing has been undertaken. The sweptback of the wing and the screen was 45o.The potential for the controllability of this flow was also evaluated by using spanwise uniform periodic excitation in order to assess the sensitivity of the flow to oblique imposed perturbations. The previous experiment (Suehiro (2017)) using splitter plate with the trailing edge having a “Λ” shape showed the complicated pattern of interacting waves emanating from both sides of the “Λ” notch. In order to investigate only one-sided effect which is simpler because of the absence of these interferences, the skewed swept-back wing was used for this research. Mean velocities measured by an array of pitot tubes, instantaneous velocities measured by PIV, and the hotwire data for spectrum analysis were taken to analyze the mixing layer. The velocity profiles with forcing frequency of 50Hz attained self-similarities whereas that with forcing frequency of 75Hz was distorted and they were not self-similar. Traditionally the center of the mixing layer is arbitrarily defined as the location where the velocity is the average of the two streams. Forcing the mixing layer at a frequency of 50Hz generated only minor distortions to the sinusoidal oscillation of the mixing layer center while forcing 75Hz distorted the center much more. The mixing layer with forcing frequency of 75Hz where the amplitude of the oscillations was 1.5mm was too amplified. As a result of the excessive amplification, there were two regimes. The flow experienced those regimes in time and in space. That is why a single scale length scale such as the vorticity thickness or the momentum thickness cannot be used to scale width of the turbulent mixing layer at all times and locations.